In the production of seamless tubing, one of the intermediate operations involves passing of a pierced tubular shell through a plug mill to enlarge the internal diameter of the shell, reduce its wall thickness and increase its length. In a typical plug mill operation, a pre-heated tubular shell is delivered to a feeding trough on the upstream side of the plug mill and advanced by a suitable pusher arrangement until the leading edge of the shell is engaged by the working rolls of the mill. The shell is then drawn by the working rolls over a mandrel, which carries a plug of appropriate dimensions at its upstream end. Typically, two passes through the plug mill are required. Thus, after the shell has passed once through the mill, the mill rolls are opened slightly and the shell is engaged by return rolls and directed back through the mill, in a non-working pass. At some point, the shell is rotated 90.degree. and then directed through a second working pass of the mill. Prior to the return or non-working pass, the original mandrel plug is caused or permitted to fall away from the pass line; a new plug is installed on the mandrel in advance of the next working pass.
As will be appreciated, when a shell is returned to the upstream side of the mill for a second working pass, it returns at greater length than when initially delivered to the mill as a result of elongation during working. Likewise, when the shell is returned after the second pass, it has been elongated still further. In conventional plug mill systems, this elongation is accommodated by positioning the shell pusher apparatus in such manner that, with the pusher ram retracted, the elongated shell, after the second pass, can be accommodated between the retracted ram and a predetermined load position in front of the mill. However, this requires that the ram be designed with a stroke capacity sufficient to push an unworked blank into the mill while at the same time being able to accommodate on the return a blank which has been elongated by as much as, say, 30%. While such an arrangement can serve to perform the intended functions, the necessarily large stroke capacity of the pusher ram results in the movable mechanisms being quite heavy and correspondingly relatively slow operating.
With conventional shell pushing systems, the problem of accommodating shell length variation is compounded by the fact that there may be significant overall length variation in the incoming pierced shells. Accordingly, the pusher system must be adaptable to accommodate not only variations in shell length resulting from elongation during the working passes, but it also must accommodate the entire range of the shortest shell, prior to elongation, to the longest shell after elongation. In a typical mill, this can be an extreme variation from, say, 3,800 mm minimum incoming shell length to, for example, 18,000 mm length in the maximum length shell after elongation.
In view of the impracticability of constructing a shell pusher ram of adequate length to accommodate the entire range of sizes, from the shortest unworked shell to the longest shell after elongation, it has been proposed in the past to locate the ram cylinder at the most remote position from the mill, to accommodate the shell of greatest length after elongation. Ram attachments are provided, which can be selectively installed on the forward end of the ram, in order to accommodate workpieces of shorter length. Among the disadvantages of this arrangement are that, when working with shells of short length, the moving parts of the ram, including the extensions, combine to constitute an extremely large mass, which is difficult to move rapidly with a ram of reasonable power capacity. In addition, the use of ram extensions still requires the ram to have an operation stroke which is excessively long, in relation to the requirements of the present invention, because there is no opportunity to change ram extensions during the working pass of a shell, so that the ram capacity has to accommodate both the initial and elongated length of a shell.
In accordance with the present invention, an improved form of shell pusher apparatus is provided, which enables the working stroke of the pusher ram to be maintained at a practical minimum, sufficient merely to advance a shell from its load position into working engagement with the mill. In conjunction with the foregoing, the minimum stroke pusher ram is mounted on a movable ram carriage which can be quickly re-positioned, not only to accommodate shells of different original length, but also to accommodate the elongation of a shell during a working pass. Thus, after initially pushing a shell into the mill, the short stroke ram is retracted, and the ram carriage is bodily moved back to a new position to accommodate the anticipated elongation of the shell. When the shell is returned to the upstream side of the mill, either for a second pass or to be discharged, the ram carriage is in an appropriate position to accommodate the then greater length of the shell. Likewise, when a shell has completed its final pass, and has been returned to the upstream side of the mill and discharged, the ram carriage can be quickly re-positioned at a location appropriate to the length of the next-loaded, unworked tubular shell.
In a most advantageous form, the shell pusher ram arrangement incorporates a hydraulic shock absorbing system, which becomes operative near the retraction limit of the shell pusher ram. This enables the ram head to serve effectively as an energy absorbing abutment stop for tubular shells being returned in the upstream direction after a working pass. In this respect, consistent with the overall objective of increasing the speed and efficiency of operation of the plug mill, it is desired that tubular shells be returned as rapidly as practicable after completion of a working pass. By utilizing the pusher ram to absorb the kinetic energy of the returning shell, higher return speed may be accommodated without damage to the end of the shell.
High speed re-positioning of the ram carriage is achieved most advantageously by an elongated rack structure, which cooperates with a heavy-duty, electrically driven pinion arrangement. In advance of each shell pushing operation, and also in advance of the return of a shell after a working pass, the drive motor is controllably energized to re-position the ram carriage at high speed. When the ram is properly positioned, the rack and pinion mechanism is locked, so that the pusher ram is firmly anchored in its appropriate location.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment and to the accompanying drawings.